GROUND FAULT PROTECTION - GFP
Ground faults in any power distribution system require special consideration. A ground fault occurs
whenever a phase conductor develops an electrically conductive path to ground. This path may be
caused by deteriorated insulation, mechanical damage, surges, heat, aging, the presence of
rodents and the intrusion of foreign objects such as tools into bare bus compartments.
A ground fault may develop as a “bolted fault” and or “arcing fault.” Ground fault protection will
protect against either type if properly set.
A “bolted fault” occurs when a phase conductor is solidly connected to ground. This produces a
high fault current which dissipates energy throughout the distribution system.
An “arcing fault” occurs when a phase conductor develops an electrically conductive path to ground,
but is not solidly connected to ground. This path develops an arcing fault which, if self-sustaining,
is capable of releasing tremendous energy at the point of fault. This tremendous energy, released
at low current levels over a period of time, creates devastating results sufficient to ignite fires.
The I.E.E.E. studies indicate that the typical arcing fault current is only 38% of the bolted fault
current. It is this low current level that renders normal over current protection useless. The only
recourse is to add supplemental relaying to secure adequate protection.
The National Electric Code requires in Article 230-95: “Ground fault protection of equipment shall
be provided for solidly grounded Wye electrical services of more than 150 volts to ground, but not
exceeding 600 volts phase to phase for each service disconnecting means rated 1,000 amperes or
more”.
The voltage range for ground fault protection is specified since arcing faults in circuits less than 150
VAC to ground are usually self-extinguishing and arcing faults in circuits over 600 VAC line to line
approach bolted fault levels which may be detected by ordinary phase over current devices if the
system is solidly grounded.
When a ground fault is detected, the Normal and/or the Emergency source will be opened,
isolating the ground fault from the external voltage source. To insure that the alternate source will
not be closed to a known fault. The Automatic Transfer Switch will be inhibited from transferring to
the alternate source until:
1) the fault is cleared
2) Automatic Transfer Switch is manually reset and
3) Ground Fault Relay is reset by pushing the Reset Pushbutton
The maximum setting of the ground fault protection shall be 1200 amperes, and the maximum time
delay shall be one second for ground fault currents equal to or greater than 3000 amperes.
For Delta connected systems, please consult the factory. Ground Fault Protection is not
required for a continuous industrial process where a non-orderly shutdown will introduce
additional or increased hazards.
Lake-Shore-Electric.com
Ground Fault Protection Accessory - 100115
800-225-0141